Recently, there has been a steady stream of new data communication standards to increase the cooperation between small electronic wireless devices, also denoted as wireless nodes. In line with this increased demand, a number of wireless technologies have been developed, including for example Bluetooth, ZigBee, wireless USB and similar technologies, that typically permit communication within about 10 meters—in other words, a very short range. These short range private ad-hoc networks are commonly denoted as wireless personal area networks (WPANs) or wireless sensor net.
The objective with a WPAN is to facilitate seamless operation among home or business devices and systems, and every wireless node in a WPAN will be able to wirelessly connect to any other wireless node in the same WPAN, provided they are within wireless transmission range of one another or can communicate via other nodes in the same network. Applications include for example cable-free connections for alarms, appliances and entertainment systems.
An example of a suitable network protocol for a WPAN is the IEEE standard 802.15, where for example Bluetooth is included (IEEE 802.15.1), which is suitable for low-power digital radios such as wireless headphones connecting with cell phones via short-range radio. Another example is the ZigBee specification (IEEE 802.15.4), which is targeted towards industrial and building automation. The ZigBee technology is intended to be simpler and cheaper than e.g. Bluetooth, and thus targets radio-frequency (RF) applications that require a low data rate and long battery life. A problem with WPAN is however that the current implementation of the network topology lacks knowledge of the accurate distance between the different wireless nodes or that the means to determine the distance are specific to the chosen protocol and in some cases not included as a compulsory requirement for the wireless nodes, thus leading to interoperation problems for position related services.
In trying to solve this problem, a method for determining the relative position of a wireless node in a wireless network has been provided by US 2005/0099333, disclosing a method for determining the distance between a base station and a mobile object. The key feature of the disclosed method is to determine differences in carrier phases being backscattered to the base station from the mobile object, where the difference will be used for determining the distance between the base station and the mobile object. However, this method uses technology solely suitable for mobile to base station communication, and will thus not be useful in relation to a WPAN. Furthermore, the method disclosed has no means to distinguish backscattered radiation from the mobile device from the abundance of passive metallic reflectors in a typical WPAN/wireless sensor network environment.
There is thus a need for an improved method for determining a distance between different wireless nodes in a short range ad-hoc network system such as a WPAN.